FIG. 8 shows round threads prescribed, for example, as a threaded joint for oil well pipes by API, and used widely. The round threads are machined by rounding crests of line pipe threads (triangular threads) used conventionally, and changing the taper of the threads from 1/32 to 1/16, in order
(1) to improve the finishing accuracy of thread cutting,
(2) to reduce the stress concentration which acts on the threads,
(3) to improve the leak resistance, and
(4) to reduce the danger of damaging the threaded portion during the work of handling, transporting and stabbing.
That is, the round threads are designed as follows. Opposite flanks of each thread abut against flanks of a corresponding thread simultaneously, so that the thread clamping force increases. As the clamping force increases more, the contact pressure on each of the flanks of the thread becomes higher so that the air tightness becomes higher. However, when tensile force is applied longitudinally to a pipe provided with these threads, there is a problem of so-called jump-out that the threads are detached in the case where an oil well becomes deeper, because the angle of load flanks loaded with the tensile force is large as 30.degree..
FIG. 9 shows a buttress-threaded (trapezoidal-threaded) joint, which was developed in order to prevent jump-out caused by increase of the depth of an oil well. The angle of each load flank loaded with tensile force (load flank angle) is 3.degree. so that the buttress-threaded joint is great in the joint strength and effective in the work of stabbing, or the like. However, these threads are characterized in that they have a disadvantage in leak resistance against internal pressure though the joint strength is great.
That is, in the buttress threads, grease charged into gaps which are produced between male and female threads when the male and female threads are fitted to each other, and pressure generated on each contact surface of load flanks and pin roots in the thread flanks and roots by make up of the threads (hereinafter referred to as "contact pressure") have a function of sealing the pressure of oil or gas applied on the internal surface of the pipe. Therefore, basically, as the fastening force of the threads increases, the above-mentioned contact pressure becomes high, and the leak resistance is improved. However, as shown in FIG. 10, when the contact pressure increases on the load flank which is a side surface loaded with tensile force, the threads are shifted in the longitudinal direction of the pipe because there is a gap C on the rear-side surface (stabbing flank) of the thread, so that the contact pressure of the load flank cannot become sufficiently high. That is, as is apparent from the graph of FIG. 11 showing the state of generation of contact pressure on each element of a thread when making up the buttress threads, though contact pressure must be generated on the load flanks over the whole length of a buttress threads, contact pressure is generated rather on the stabbing flanks in the small-diameter male threads. This means that the threads move in the longitudinal direction of the pipe.
Background-art threads for oil well pipes according to API as described above are tapered threads, in which make up of the threads results in that not only contact pressure on thread flanks is increased by the wedge effect so that the leak resistance is improved, but also tensile stress (hoop stress) is generated in the circumferential direction of a coupling. In addition, in the case of the buttress threads which have a large permissible tolerance in the make up position, there is a problem that hoop stress exceeds a yield strength of the material in the worst case. Further, since the above-mentioned sealing performance of the buttress threads depends on the grease charged into gaps between the threads, there is another problem even though the sealing performance is proof against the pressure of liquid, gas leaks easily because of small molecules of the gas.
In order to improve such a sealing function of the buttress threads, a joint extremely superior in the sealing performance was developed by providing a metal seal portion in which a tapered outer surface and an end surface formed in a front end portion of male threads are pushed with pressure against a tapered inner circumferential surface and a stepped portion formed in a rear end portion of female threads. However, since the metal seal portion must be high in the dimensional accuracy, there is a problem that it is difficult to finish the metal seal portion.